The present invention relates to a semiconductor laser device suitable as the light source for an optical disc or the like.
A prior art semiconductor laser device will be explained in the following.
FIG. 15 shows a cross-sectional illustration of a prior art semiconductor laser device (refer, for example, to Japanese patent placed to open public S62-73687).
On an n-type gallium arsenide (GaAs) substrate 21 are formed successively an n-type gallium aluminum arsenide (Ga.sub.0.65 Al.sub.0.35 As) cladding layer 22, an active GaAs layer 23 and a first p-type Ga.sub.0.75 Al.sub.0.25 As cladding layer 24.
Then, an n-type Ga.sub.0.51 Al.sub.0.49 As current blocking layer 25 is formed on the first cladding layer 24 for pinching current except for an area where a window 25a for serving as a current channel is formed. Item 26 is a p-type Ga.sub.0.75 Al.sub.0.25 As second cladding layer formed in a regrowth process and item 27 is a p-type GaAs contact layer.
In the structure of FIG. 15, a current injected to the semi-conductor laser at the p-type GaAs contact layer 27 is confined in the window 25a effectively and the laser oscillation occurs in the GaAs active layer 23 under the window 25a.
At this time, the refractive index of the n-type Ga.sub.0.51 Al.sub.0.49 As current blocking layer 25 is smaller than that of the p-type Ga.sub.0.75 Al.sub.0.25 As second cladding layer 26 and the laser light is also confined in the window 25a effectively.
Further, since the energy gap of the n-type Ga.sub.0.51 Al.sub.0.49 As current blocking layer 25 is sufficiently larger than that of the GaAs active layer 23, therefore, the n-type Ga.sub.0.51 Al.sub.0.49 As current blocking layer 25 will become transparent for the laser light and a semiconductor laser device of small internal loss and low operating current will be obtained. However, it has been difficult to obtain laser light of the 780 nm band.
Since a regrowth process is difficult with the foregoing prior art semiconductor laser device, it has been impossible to increase the AlAs mole-fraction in the p-type Ga.sub.0.75 Al.sub.0.25 As first cladding layer 24 with resultant unsatisfactory temperature characteristics.
In other words, when a regrowth processing is applied to GaAlAs of a high AlAs mole-fraction, crystallization at the regrowth surface is deteriorated due to the surface oxidation and defects in the current-voltage characteristics will be caused with a resultant necessity of lowering the AlAs mole-fraction of the first cladding layer 24 to some extent. For an easy production, it is necessary to make the AlAs mole-fraction to be less than 0.3.
However, there exists a limit in confinement of carriers into the active layer 23 and a semiconductor laser device of excellent temperature characteristics cannot be obtained. Particularly, it will be extremely difficult in this case to realize a visible laser oscillation which requires an AlAs mole-fraction of around 0.5.
Besides, a time control etchant is used in the etching process of the stripe-like area and it is difficult to suspend the etching process within the small limit of the thin first cladding layer 24, resulting in a reduction of the yield rate.
In addition, it is difficult to realize low noise characteristics required by applications of an optical disc or the like. The reason is that the AlAs mole-fraction of the first cladding layer 24 needs to be set low according to the aforementioned reason and the effective refractive index step in the direction parallel to the junction plane will become large, causing the spectra to be a single mode.